Due to increasingly stringent environmental regulations that limit vehicle emissions, increased fuel efficiency has become a critical requirement for vehicle manufacturers. In recent years, lubricants have been formulated to help deliver a portion of the fuel efficiency mandated by governments, required by equipment builders, and desired by end customers. A proven approach to enhancing lubricant-derived fuel efficiency is to lower the viscosity of the lubricant. This, however, generally means that there are thinner oil films between moving parts. Since equipment durability cannot be compromised, the same or lower viscosity lubricants must deliver improved efficiency while retaining the same level of protection against various types of hardware damage (wear, micropitting, macropitting, scuffing, etc).
In automobile axles and transmissions, the fuel economy benefit is determined by the sum of viscous and traction effects. Fixed losses, which respond to speed, include losses due to lubricant churning, shaft bearings and seals. Generally, these fixed losses are impacted by the lubricant viscosity, such that higher speeds and lower loads favor use of lower viscosity lubricants. Contact losses, which respond to load and speed, include gear meshing. Generally, these contact losses are reduced by using a lubricant with lower traction coefficient, which can be viewed as lower internal friction between molecules of the lubricant under high load conditions.
Accordingly, there is a need for a lubricant that delivers lower traction and friction coefficients than conventional base oil/VM technology in a typical gear oil formulation, while maintaining or improving wear and load-carrying performance. The present invention satisfies this need by providing novel combinations of base stocks that give the desired performance. Additionally, the lubricants of the present inventions provide improved low temperature flow properties, which contribute to potential efficiency gains, and improved shear stability, which contributes to increased oil life and oil film durability. The present invention also provides methods for improving shear stability, wear and load characteristics in a lubricating composition.
Air entrainment is another issue in lubricating oils. All lubricating oil systems contain some air. It can be found in four phases: free air, dissolved air, entrained air and foam. Free air is trapped in a system, such as an air pocket in a hydraulic line. Dissolved air is in solution with the oil and is not visible to the naked eye. Foam is a collection of closely packed bubbles surrounded by thin films of oil that collect on the surface of the oil.
Air entrainment is a small amount of air in the form of extremely small bubbles (generally less than 1 mm in diameter) dispersed throughout the bulk oil. Agitation of lubricating oil with air in equipment, such as bearings, couplings, gears, pumps, and oil return lines, may produce a dispersion of finely divided air bubbles in the oil. If the residence time in the reservoir is too short to allow the air bubbles to rise to the oil surface, a mixture of air and oil will circulate through the lubricating oil system. This may result in an inability to maintain oil pressure (particularly with centrifugal pumps), incomplete oil films in bearings and gears, and poor hydraulic system performance or failure. A partial list of potential effects of air entrainment include: pump cavitation, spongy, erratic operation of hydraulics, loss of precision control, vibrations, oil oxidation and component wear due to reduced lubricant viscosity.
One widely used method to test air release properties of petroleum oils is ASTM D3427. This test method measures the time for the entrained air content to fall to the relatively low value of 0.2% under a standardized set of test conditions and hence permits the comparison of the ability of oils to separate entrained air under conditions where a separation time is available.
In the ASTM D3427 method, compressed air is blown through the test oil, which has been heated to a temperature of 50° C. After the air flow is stopped, the time required for the air entrained in the oil to reduce in volume to 0.2% is usually recorded as the air release time.
Accordingly, there is also a need for a lubricant that provides favorable air release and foam properties. The present inventions satisfy this need by providing novel combinations of base stocks that give the desired performance. The present inventions also provide methods for improving air release and foam collapse rate in a lubricating composition.